Of 8,093 participants included in the study, 62 per cent had deployed to Iraq or Afghanistan.

Among those who deployed to the conflicts, the rate of probable PTSD for veterans was nine per cent compared to 5 per cent for veterans who did not deploy.

However it was far higher among ex-serving personnel who deployed in a combat role to Iraq or Afghanistan, 17 per cent reported symptoms of PTSD compared to 6 per cent of those in a support role such as medical, logistics, signals and aircrew.

Senior author Professor Sir Simon Wessely, Regius Professor of Psychiatry at King’s College London, said: “Our results suggest the risk of mental ill health is carried by those who have left the service, and that part of the legacy of conflicts on mental health has taken time to reveal itself.”

The Help for Heroes' survey of 189 veterans found that 28 per cent of respondents did not seek help due to believing civilian services would not understand or support them , while a quarter thought they would be treated differently by their friends.

The campaign coincides with a new support programme by Veterans' Gateway, a service funded by the Ministry of Defence, to help vulnerable ex-servicemen and women.

Veterans' Gateway is trialling an outreach service to phone ex-service personnel who have previously contacted its helpline.

The scheme, paid for with £108,000 funding from the MoD, is based on the US Marine Corps veterans service, which makes six proactive calls for every one it receives.

Mark Collins, assistant director of Veterans' Gateway, said: "We will be monitoring the outcome to see how this trial impacts our users but hopefully it'll mean those most vulnerable will be able to access help from Veterans' Gateway supporting organisations, on their journey to getting the right help."